The stability of actively growing heterokaryons made by fusing haploid and diploid plasmodia of has been investigated with the aid of genetic markers affecting plasmodial colour and amino acid requirements. All heterokaryons initially expressed the dominant alleles present in both component plasmodia, but after a few subcultures every heterokaryon synthesized from a haploid plus a diploid plasmodium changed to express the recessive alleles carried by the haploid component. In contrast, heterokaryons synthesized from combinations of haploid plasmodia remained stable throughout many subcultures. No evidence was found for the segregation of incompatibility alleles responsible for heterokaryon instability, although the possibility could not be excluded that a gene closely linked to was involved. A direct test of the effect of ploidy on heterokaryon stability was carried out, in which the use of isogenic haploid and diploid plasmodia allowed the effect of incompatibility genes to be eliminated. Again the phenotype of every haploid plus diploid heterokaryon changed to that of the haploid component, whereas haploid plus haploid heterokaryons remained stable. Variations in the relative sizes of the haploid and diploid plasmodia altered the speed, but not the direction, of the changes observed. Analysis of progeny indicated that the changes in phenotype were due to loss of diploid nuclei from the heterokaryons.


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